In the simplest case the hose is a two-layer hose without embedded reinforcement layer, that is, composed exclusively of an internal layer and an external layer. The duplex extrusion process is relevant in this context. However, the arrangement mostly has, between the internal layer and the external layer, a reinforcement layer composed of one or more sublayers. There can also be further layers present alongside the main two-layer structure and the reinforcement layer.
Because the internal layer with barrier layer function with respect to the medium to be transported requires solvent resistance and also high temperature resistance, the internal layer is preferably made from a fluoro rubber mixture. The crosslinking in these mixtures, known as FKM mixtures, and also as FPM mixtures, can use polyols and quaternary ammonium salts, described as by way of example in U.S. Pat. No. 5,356,681 or else in U.S. Pat. No. 6,340,511, or bisphenols and/or peroxides, described as by way of example in EP 1 396 670 B1.
An important consideration in the use of fluoro rubber mixtures, particularly in the use as internal layer in hoses, is that the rubber mixture has good tear-propagation resistance.
The internal layer is intended to function as barrier layer with respect to aggressive media, for example acids or fuels, and great importance is therefore attached to the tear resistance thereof, in particular the tear-propagation resistance.
A conventional method for increasing tear-propagation resistance is controlled undercrosslinking of the mixture. The method here is that in the case of mixtures crosslinked by bisphenols the content of bisphenol is reduced, and in the case of mixtures crosslinked by peroxides the content of the organic peroxide and/or the content of co-crosslinking agent, for example TAIC, TAC, or TRIM, is reduced. However, this leads to significantly reduced tensile strengths. DE 10 2011 055 316 A1 has already disclosed that the use of active fillers in fluoro rubber mixtures leads to improved tear-propagation resistance. However, addition of active fillers, in particular active carbon blacks, to fluoro rubber mixtures raises the viscosity, and thus impairs the processing of the rubber mixture.